After a gas turbine engine, such as an aircraft engine, is shut down, fuel left in the nozzles and nearby plumbing can overheat and cause coking. Fuel left in the nozzles can also leak out of the engine onto the ground or tarmac. The coking deposits are detrimental to the engine and may affect engine performance, and the leaking of fuel onto the ground or tarmac is prohibited by federal regulations.
To address these issues, many modern gas turbine engines incorporate a fuel ecology system to purge the fuel manifolds after shutdown, often by using an “ecology valve” that withdraws a quantity of fuel from the manifolds. This both reduces the formation of coking deposits in the manifolds and nozzles and prevents fuel from dripping out of the engine after shutdown.
Particular ecology systems for gas turbine engines have previously been described in: U.S. Pat. Nos. 6,314,998; 6,334,296; 6,385,962; and 6,422,021, each titled “Fuel Divider and Ecology System for a Gas Turbine Engine,” the entire contents of which are herein incorporated by reference. Each of these patents describes an arrangement having a plurality of engine fuel manifolds and a fuel dividing arrangement for distributing fuel flow among the plurality of manifolds.
One known fuel ecology system utilizes a piston to draw the fuel backwards from the fuel nozzles to a storage space. The piston is biased into a first position by pressure in the fuel supply path, which is communicated to the piston via a first fuel line. When the pressure in the fuel supply path drops at engine shutdown, the biasing force is removed from the piston allowing it to shift to a second position under the force of a biasing spring. This movement from the first to the second position draws fuel from the manifold. The volume of fuel drawn from the fuel manifolds in this particular system is approximately 7.8 cubic inches.
At the next engine start, fuel is pumped to the fuel manifolds and through the first fuel line to the ecology valve. The 7.8 cubic inches of fuel is re-introduced to the manifolds as rising fuel pressure drives the piston toward the first position. However, at a typical start flow rate of 160 pph, a system equipped with an ecology valve requires nearly 5 seconds to fill the fuel manifolds and deliver fuel to the engine combustion chamber.
Ideally, fuel should be available in the combustion chamber to start the engine when the engine reaches 10 to 15 percent of maximum speed; such an engine may be referred to as having a lightoff speed envelope of 10 to 15 percent. Unfortunately, in fuel delivery systems incorporating an ecology valve, the starter may accelerate the engine to a speed higher than the lightoff speed envelope, to about 22 percent of maximum speed in some cases, before the fuel manifolds are adequately pressurized. Engines may not start reliably at speeds outside the lightoff speed envelope. It is therefore desirable to provide a fuel control system with an ecology function that can be started while engine speed is within the engine's lightoff speed envelope.